This invention relates generally to electronic devices, and, particularly, relates to means for mounting chip carriers to printed wiring boards.
In the past, integrated circuit chips have been prepared wherein such a chip usually includes a plurality of active elements such as transistors, resistors, capacitors and the like, intercoupled in an integral manner on a single silicon chip. Each chip is packaged individually in containers such as cans or flat-packs or carriers, or mounted directly to hybrid packages. Individual leads are connected to the packaged chip carrier for connection to exterior circuitry.
Hermetic chip carriers, hereinafter referred to as carriers, come in package sizes having twenty eight solder pads or more thereon for connecting to external circuitry which can be on, for example, printed wiring boards.
In the past, after these carriers have been soldered onto the boards, the solder joints thereformed when subjected to temperature cycling from -55.degree. C. to about 125.degree. C. have experienced cracking, and as a result therefrom, electrical discontinuities occurred.
Solder cracking is caused by coefficient of thermal expansion differences between the carrier and the board. This mismatch between the carrier and the board causes a different growth of both. Problems still prevail even if the board material is the same as the carrier. The board is limited in size because of the brittleness of the alumina used thereon. Secondly, there is normally a power-on condition on the carrier. This power-on condition causes a thermal gradient across the solder joints from the carrier to the board. Some joints will expand more than others resulting in localized stresses and thus cracking.
One method used in the past to solve this problem was an elastic, electrically conductive joint, but this has resulted in the carrier becoming thermally isolated and thus overheating.
The present invention is directed toward providing a means of mounting the carrier to the board in which these undesirable characteristics are minimized.